1. The Field of the Invention
The present invention pertains to a method and system for on site water purification treatment and, in particular, to a unit which uses a cascade ion exchange system to produce high purity water from a local water supply.
2. The Prior Art
There are a number of well known and effective means for total removal of impurities from water. For example, one technique uses flash evaporators, but these require a substantial capital investment. The invention of modem ion exchange technology has made it possible to produce high purity water without requiring the high capital expenses previously associated with flash evaporators. Thus flash evaporation, in many cases, is no longer considered to be an economical alternative for the provision of high purity water, especially for lower volume applications. Membrane separation, commonly known as reverse osmosis separation, has gained a great acceptance in the past few decades. It is capable of removing more than 95% of the dissolved solids from water feed streams in a single stage. The two-bed ion exchanger (a Strong Acid Cation (SAC) resin bed followed by a Strong Basic Anion (SBA) resin bed) is a common ion exchanger arrangement used in water purification systems. Multiple arrangements of strong and weak cation and anion beds are valid options for specific water treatment applications, but they are not suitable for all applications.
The purity of demineralized water is measured by the specific electrical resistance of the water, measured in ohms, to an electric current. Electric current has less resistance in the presence of high concentrations of electrolytes (dissolved ionized solids) than in low concentration solutions. Water with a specific resistance of 100,000 ohms-cm has an equivalent Total Dissolved Solids (TDS) of 2-3 ppm. Such water is considered "pure" and could be produced by a two-bed ion exchanger. Water with a specific resistance of 1,000,000 ohms-cm (1 Meg ohm-cm) has an equivalent TDS of 0.2-1.0 ppm. Such water is considered "very pure" and could be produced by a four-bed ion exchanger (two two-bed ion exchangers in series). A mixed bed ion exchanger is capable of producing water with a specific resistance of 2,000,000 ohms-cm (equivalent to 0.2-0.5 ppm TDS). A two-bed ion exchanger in series with a mixed bed is capable of producing water with a specific resistance of 10,000,000 ohms-cm (equivalent to 0.04-0.1 ppm TDS). This water is considered to be "ultrapure." The highest water purity that is practically achievable would have a specific resistance of 18,300,000 ohms-cm (18.3 Meg ohms-cm).
The invention of the mixed ion bed in the early 1950's (see U.S. Pat. Nos. 2,578,937 and 2,692,244) had a dramatic effect in the art of water purification. Mixed bed ion exchangers can produce ultrapure water, defined above as water with a specific electric resistance of 18 Meg ohms-cm (18,000,000 ohms-cm). However, mixed bed ion exchangers are less efficient than single ion bed exchangers and the mixed ion bed can be quickly exhausted. Therefor, conventional two bed ion exchangers, or reverse osmosis membranes, are usually used for a pre-treatment of the water fed to the mixed bed ion exchange.
The patents issued to Casolo, Boeve, Berry, Mahmud and Miyamura, discussed below, are directed to inventions for producing high purity water. All of the disclosed inventions include: known filtration means to remove undissolved impurities in the form of sediments; known adsorption means to remove colloidal and organic matters; and known ion exchange means to remove ionic impurities. In most of these inventions, mineral purification is accomplished in a two-step process utilizing conventional two or four bed ion exchange systems followed by mixed bed exchanges for final purification of the water. Berry proposes two mixed beds in series.
U.S. Pat. No. 3,985,648 to Casolo has, as an object, to optimize the efficiency of an ion exchange by adjusting the pH of the water as it passes through the different treatment units. A four (4) bed system plus a mixed bed and chelation resin were used. The bed arrangement was as follows: Strong Acid Cation followed by Weak Base Anion followed by Weak Acid Cation, followed by Strong Base Anion (SAC, WBA, WAC, SBA) and finally the mixed and chelation beds. The novelty in this arrangement is the addition of the Weak Acid Cation bed. The three-bed arrangement SAC, WBA, SBA is known from the prior art. In this arrangement the WBA (unit 28) neutralizes the acidic water coming from the SAC (unit 24). This neutral or high pH water enters the WAC (unit 32) and improves the efficiency of this bed. In the meantime, the water leaving the WAC (unit 32) has a low pH, which in turn improves the efficiency of the subsequent SAC (unit 36) in removing heavy metals. The mixed bed and the chelation bed are also known in the prior art. This system is a closed loop with no water discharge being allowed, for fear of contaminating waterways. Regeneration in place was not considered and no explanation is given as to how regeneration is to be accomplished or how to dispose of the accumulated industrial waste. This is a noticeable deficiency to this invention.
U.S. Pat. No. 4,548,716 to Boeve has, as an objective, to produce pyrogen free, ultrapure water with specific electrical resistance of at least 18 meg ohms-cm. The emphasis is on using pure ozone at concentrations of two mg/l in an ozone contactor to insure destruction of pyrogen. The system is capable of processing water from different sources. A relatively large collection of treatment techniques is used including coarse filtration, carbon adsorption, membrane separation, dual and mixed bed deionization and several disinfection units (three ultraviolet units and one ozone unit). All operations of this invention are individually known in the prior art.
U.S. Pat. No. 4,280,912 to Berry has, as an objective, to produce ultrapure water with a specific electrical resistance of at least 18 meg ohms-cm at the point-of-use. The system consists of all standard unit operations outlined above. In addition, it includes means to monitor purity up stream and down stream of the water purification system; to shutdown the system upon detecting specific resistances below 500,000 ohms-cm; and to measure and control water temperature. A final 0.2 micron filter is used at the dispensing spout. Current operation conditions imply that a pure water supply (which could be a dual or three-bed pretreatment deioniser) is used to feed this system.
U.S. Pat. No. 5,024,766 to Mahmud has, as an objective, to maintain the purity of an ultra pure water source at a low Total Organic Carbon (TOC) and bacteria at point-of-use. A purified water source with specific resistance of 17-18 meg ohms/cm is required. The point-of-use system includes a circulation loop that contains a pump, an ultraviolet unit, a 0.2 micron filter, a twin mixed bed polisher and an ozone generator sterilizer.
U.S. Pat. No. 5,259,972 to Miyamura has, as an objective, to produce, for the electronic industry, ultra pure water with a specific resistance of 10 meg ohms/cm or more and 10 parts per billion of nonvolatile residue. Removal of colloidal substance by oxidation with ozone into fine particles of oxides that can easily be removed and do not clog the reverse osmosis membrane is a primary goal. In addition to the conventional purification elements, a reverse osmosis membrane is used as a last filtration element to remove particles that can't be removed by ion exchange.
U.S. Pat. No. 5,190,659 to Wang has, as its object, to provide a point-of-entry (POE) water conditioning system. The proposed system will partially treat water contaminants. It is not intended to produce high purity water. The primary components are the vessels for filter media (a wide array of reactive and regenerative compounds that are intended to treat specific problems are disclosed), a process tank for regeneration and chemical circulation, a central flow control (without a description of how it works), a pump and ultraviolet units.
U.S. Pat. No. 5,259,975 to Mohn has, as an object, to complex and stabilize waste water metal hydroxide bearing sludge with tolyl triazole to inhibit leaching of metal from the sludge into the environment.
A two-bed cation/anion ion exchanger is a relatively simple device, but it does not achieve the high level of water purity required by some technologies, such as the electronic and pharmaceutical industries. On the other hand, the known mixed bed devices produce very high purity water but they are less efficient and require cumbersome means to segregate, regenerate and remix the ion exchange resin.